1. Field of the Invention
The present invention relates to an LED pulse shaping circuit and, more particularly, to a variable switched circuit capable of improving both the rise time and fall time of an LED.
2. Description of the Prior Art
The LED has emerged as a promising optical source for applications such as local area networks (LANs), computer networks, and interoffice communication system which are less than about 3 km in length. Interest in utilizing LEDs is primarily due to their lower cost versus laser-based alternatives. For example, employing high power LEDs in transmitters greatly relaxes the alignment tolerances for the optical assembly which coupled to the LED to the communication fiber. Unfortunately, high power LEDs, especially long wavelength devices, are usually unsuitable for high speed applications (i.e., &gt;100 Mb/s due to their relatively slow rise and fall times (typically &gt;3 ns).
A solution to the slow response time problem of these LEDs is to include a pulse-shaping circuit in the LED trasmitter. Pulse shaping circuits generally perform two functions: current peaking and or charge extraction. Current peaking on the rising edge of the LED drive current pulse shorts the optical rise time by quickly charging the space charge capacitance C.sub.s of the LED and thus eliminates the need for a DC prebias (which can degrade the extinction ratio). Charge extraction on the trailing edge of the drive current pulse forces a large reverse current through the LED which discharges the diffusion capacitance D.sub.d. This reverse current, applied for a relatively short period of time, rapidly sweeps out the remaining injected carriers instead of allowing them to recombine, thus reducing the tail on the optical falling edge.
U.S. Pat. No. 4,166,947, issued to J. P. Dirksen on Sep. 4, 1979 is illustrative of the conventional techniques used to improve the fall time of LEDs. Dirksen discloses resistive shunting of an LED in response to a turn-off signal to provide a low impedance path for the elimination of the minority carriers in the device. An alternative arrangement is disclosed in U.S. Pat. NO. 4,295,226 issued to L. C. Dombrtoski on Oct. 13, 1981. Dombrowski discloses a driver circuit arranged to bias the LED so as to draw current during either its ON or OFF state. The arrangement allows for the effective overdriving of current through the LED in the reverse, or negative direction. This overdriving of the LED aids the sweeping of minority carriers out of the junction region and thereby improves the fall time of the LED optical output. However, since the LED draws current during both the ON and OFF states, different levels of optical output will exist in each state, requiring additional threshold detection circuitry to determine the state of the device.
Thus, a need remains in the art for a pulse shaping circuit which s capable of improving both the rise and fall times of LEDs while overcoming the limitations of the prior art arrangements.